1. Elaidic and linoleic acids were administered to rats intraperitoneally at dose levels of 40 and 200 mg/kg during 5 weeks. Fattyacids had a tendency to decrease both the amount of protein and phospholipids in livermicrosomes. Also the fattyacid composition of the microsomal phospholipids was modified by the fattyacids.

2. Microsomal aryl hydrocarbon hydroxylase and p-nitroanisole-O-demethylase activities were lowered by linoleic acid. The decrease in the enzyme activities was, however, not identical. Also elaidic acid tended to decrease the activity of these enzymes. In addition, a slight decrease in microsomal cytochrome P-450 content and in the activity of NADPH cytochrome c reductase took place.

3. The activity of microsomal UDPglucuronosyltransferase was significantly decreased by both of the fattyacids.

4. The results suggest that unsaturatedfattyacids affect the activities of microsomal drug metabolizing enzymes at least partly by modifying the membrane structure.

5. It seems that the effects are substrate specific and to some extent opposite to those of dietary unsaturated triglycerides and cholesterol.